24 C.F.R. Appendix I to Subpart B of Part 51—Definition of Acoustical Quantities
Title 24 - Housing and Urban Development
1. Sound Level. The quantity in decibels measured with an instrument satisfying requirements of American National Standard Specification for Type 1 Sound Level Meters S1.4–1971. Fast time-averaging and A-frequency weighting are to be used, unless others are specified. The sound level meter with the A-weighting is progressively less sensitive to sounds of frequency below 1,000 hertz (cycles per second), somewhat as is the ear. With fast time averaging the sound level meter responds particularly to recent sounds almost as quickly as does the ear in judging the loudness of a sound. 2. Average Sound Level. Average sound level, in decibels, is the level of the mean-square A-weighted sound pressure during the stated time period, with reference to the square of the standard reference sound pressure of 20 micropascals. Day-night average sound level, abbreviated as DNL, and symbolized mathematically as Ldn is defined as:
Title 24: Housing and Urban Development
PART 51—ENVIRONMENTAL CRITERIA AND STANDARDS
Subpart B—Noise Abatement and Control
Appendix I to Subpart B of Part 51—Definition of Acoustical Quantities
